It's not quite "changeable." Then it would be the word "change." The word "mutate" means a variable, during its lifespan, arrives at the API from the frontend the same as it was defined. For example, a component could define default form values, like populating the input "name" with the string "John Doe". But if the form library supplies the API with an array ['john,' 'doe'] then it was mutated. This is not only based on changing types- you get the idea.